726 PAPERMAKING / Paperboard Grades Glassman A (1985) Printing Fundamentals. Atlanta, GA: pulp fiber is important, the greatest factor affecting Technical Association of the Print and Paper Industry. bending stiffness is sheet thickness or caliper. Rela- Lebtinen E (ed.) (2000) Papermaking Science and Technol- tively small changes in thickness will result in large ogy: Pigment Coating and Surface Sizing of Paper: changes in bending stiffness since the effect of sheet Helsinki: Fapet Oy. thickness on stiffness is cubic rather than linear. Pulp Oittinen P and Saarelma H (eds) (1998) Papermaking selection for greater fiber stiffness leads papermakers Science and Technology: Printing: Helsinki: Fapet Oy. to choose naturally thicker fibers such as softwood or Parker JD (1972) Sheet-Forming Process. Atlanta, GA: Technical Association of the Pulp and Paper Industry. hardwoods with noncollapsible, round cross-sections. Paulapuro H (ed.) (1998) Papermaking Science and High yield pulping processes such as thermomecha- Technology: Paper and Board Grades: Helsinki: Fapet Oy. nical pulp (TMP) or semichemical that produce pulp Scott WE and Abbott JC (1995) Properties of Paper: An with high percentages of lignin are also desirable for Introduction, 2nd edn. Atlanta, GA: Technical Associa- paperboard because of their inherently stiffer fibers. tion of the Pulp and Paper Industry. High yield pulps are very desirable for the manufac- Smook GA (1992) Handbook for Pulp and Paper ture of paperboard because the fiber is not only stiff Technologists, 2nd edn. Vancouver, Canada: Angus but produces bulky sheets that enhance bending Wilde. stiffness through both fiber stiffness and sheet thick- ness. For these two reasons, recycled fiber is also used to a large degree in paperboard manufacture. Paperboard Grades Manufacturing Multilayer Paperboard G J F Ring, University of Wisconsin–Stevens Point, Stevens Point, WI, USA The same basic paper machines used to produce writing and printing paper are also used to form & 2004, Elsevier Ltd. All Rights Reserved. paperboard. However, modern paper machines are limited in their ability to produce a single-layer paper À sheet with a grammage above 150 g m 2. There are a Introduction number of reasons for this limitation. Primarily, thicker single-layer sheets are more difficult to Paperboard is a heavyweight paper that has a dewater requiring excessive reductions in machine grammage or basis weight generally greater than speed. Furthermore, the increased drainage forces À 150 g m 2 and is thicker than 300 mm. The addi- applied to thicker sheets in the forming section tional thickness results in a stiffer sheet that is ideal would cause greater fines removal from the bottom for boxes and containers providing protection for of the sheet resulting in a rougher surface. The saleable products during transportation, handling, topside of a very thick sheet would also be adversely and storage. Paperboard grades include carton- affected since paper is formed on fourdrinier boards, containerboards, and specialty boards, and machines layer by layer from the wire side up, which are used in either food or non-food applications. The would allow extra time for the fibers in the top layer English units for paperboard are given in pounds per to flock and produce a ‘hill and valley’ appearance. 1000 square feet (lb/1000 ft2) and the thickness is The combination of these two effects would produce measured in points or mils (thousands of an inch). an unacceptably two-sided product. A box is a rectangular container with or without a Manufacturing multilayered paperboard from top designed to transport and store a variety of goods separately formed sheets provides a solution to the and products in either liquid or solid form. A box above-mentioned problems. The forming section of must be strong enough to hold its contents not only paperboard machines are composed of two, three, or during transportation and storage but also during the even four forming sections that bring individual packaging operation. Because boxes are frequently sheets together at the wet press. Paperboard stored in stacks, compression resistance in their machines are for this reason large and complex vertical sides is required to avoid bulging and having heights that are two to three times greater cracking that would result in loss or damage to the than single-former machines. Any one of the former product contained within. sections in a multilayer machine can be either a Boxes manufactured from paperboard are made to traditional fourdrinier or a modified fourdrinier resist compressive forces by using stiff pulp fibers equipped with a top-wire unit for additional dewa- formed into thick sheets. Paperboard meeting these tering capacity. The use of different furnishes in each two requirements will have high levels of bending former produces a final sheet that is engineered for stiffness. Although the stiffness requirement for the specific stiffness and smoothness requirements. PAPERMAKING / Paperboard Grades 727 Although initially forming two to three separately (FBB), white lined chipboard (WLC), solid bleached formed sheets of paper, a multilayer machine forms a sulfate board (SBS), solid unbleached sulfate board single sheet of paperboard when the individual sheets (SUS), and liquid packaging board (LPB) are major of paper are combined together in the wet press. The types of cartonboard grades. Many of these grades individual single-layered sheets prior to the wet press will be pigment coated to produce a highly printable are ‘vacuum dewatered’ with a typical consistency of top surface. These paperboards need to be strong and 20% (80% moisture) and are simply assemblages of stiff to form the walls of a carton. Because cartons fibers held together by capillary forces exerted by the are stacked during storage, compression resistance is continuous matrix of water surrounding the fibers. also important. In particular, cross-machine direction When the sheet continues it progress through the wet (CD) stiffness is required to prevent carton bulging press and the dryers, this continuous matrix of water since the carton is manufactured with the CD parallel is decreased and the fibers are progressively drawn to its height. High-speed converting and mechanized together through surface tension. Eventually, at the packaging lines mandate excellent runnability; there- end of the drying process with a final moisture fore, resistance to curl is also a necessity. Finally, content of 4–8%, the surface tension forces between folding these paperboards into containers must occur individual fibers will produce pressures sufficiently without cracking, which requires not only good high enough to form fiber-to-fiber hydrogen bonds strength, but also good stretch. The ream area for resulting in a mechanically strong sheet. During paperboard grades is 1000 ft2. multilayer forming, a single sheet of paperboard is formed from the individual sheets of paper by merging the water matrices of each sheet into a Folded Boxboard single, hydraulically connected matrix in the wet Folding boxboard grades are used to make packages press. The net result is that the multilayer sheet for numerous products including: dry foods, liquid continues through the wet press and dryer section foods, frozen foods, confectionaries, cosmetics, forming fiber-to-fiber bonds inside layers and be- detergents, and numerous other items. The gram- tween layers as if they were initially formed together. À mage range for folding boxboard is 160–450 g m 2. Theoretically, the fiber-to-fiber bonding between Folding boxboard is a multilayer sheet having three separately formed layers will be identical to fiber- to four layers. The top and bottom layers are to-fiber bonding within a single layer. Differences in frequently made of bleached chemical pulp. The interlayer bonding strength (measured by z-direction middle layers consist of either mechanical pulp or strength tests) will be found when the individual recycled fiber for stiffness enhancing bulk. The top sheets are wet pressed at moisture contents lower layer is usually surface sized for good picking than what is necessary to form a hydraulically resistance in printing. Because the board must have connected matrix. (z-direction strength is the max- a smooth top surface for good printability, yet have imum tensile force per unit area which a paper or adequate thickness for good bending stiffness, paperboard can withstand when applied perpendi- smoothness is obtained through a combination of cularly to the plane of the test sample.) limited calendaring and coating. It is important for The advantage of manufacturing a multilayer sheet folding boxboard grades to also have good folding is that key paper properties can be engineered into and scoring properties. the paperboard that would not be obtainable by single-layer forming. Special top layers can be incorporated that are white and smooth, therefore, White Lined Chipboard having excellent printing properties. Middle layers can be used that are bulky and thus inherently White lined chipboard (grammage range of 200– À thicker producing the stiffest possible board. These 450 g m 2) uses recycled fiber in its middle layers, middle layers can also contain recycle fibers or pulp and has many similar applications as folding box- fibers of lower quality that can be covered or masked board except for certain food packaging applications by higher quality top and/or bottom layers. where contaminants in the recycled pulp might spoil or taint the food. WLC grades may sometimes have Cartonboard Grades four sheet layers. The fourth layer is actually an additional top layer used to maximize use of less Cartonboard produces consumer product packaging bright recycled pulps, therefore minimizing the use of for food and retail items. The market for these grades expensive highly bleached top layers. The top layer is is large and product development is great due to frequently coated to increase printability and an dynamic marketplace forces. Folding boxboard optional bottom coating may be found. 728 PAPERMAKING / Paperboard Grades Solid Bleached Sulfate Board embossing.